Method for manufacturing semiconductor device

ABSTRACT

The number of masks and photolithography processes used in a manufacturing process of a semiconductor device are reduced. A first conductive film is formed over a substrate; a first insulating film is formed over the first conductive film; a semiconductor film is formed over the first insulating film; a semiconductor film including a channel region is formed by etching part of the semiconductor film; a second insulating film is formed over the semiconductor film; a mask is formed over the second insulating film; a first portion of the second insulating film that overlaps the semiconductor film and second portions of the first insulating film and the second insulating film that do not overlap the semiconductor film are removed with the use of the mask; the mask is removed; and a second conductive film electrically connected to the semiconductor film is formed over at least part of the second insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

2. Description of the Related Art

In recent years, transistors that are formed using a semiconductor thinfilm having a thickness of several nanometers to several hundreds ofnanometers over a substrate having an insulating surface such as a glasssubstrate have been attracting attentions. Transistors are widely usedfor electronic devices such as ICs (integrated circuits) andelectro-optical devices. In addition, transistors are used as switchingelements of active matrix display devices typified by liquid crystaldisplay devices and active matrix light-emitting devices typified bylight-emitting devices including organic electroluminescence (EL)elements.

The ranges of uses of such an active matrix display device and such alight-emitting device are expanding, and demands for larger screen size,higher definition, and higher aperture ratio are increasing. Inaddition, these devices are required to have high reliability, andproduction methods of these devices are required to provide high yieldand to have low production cost.

A reduction in the number of photolithography processes in manufactureof transistors for an active matrix display device and an active matrixlight-emitting device is important for cost reduction. For example, onephotomask for the eighth generation glass substrate costs tens ofmillions of yen, and one photomask for the tenth generation glasssubstrate or the eleventh generation glass substrate costs hundreds ofmillions of yen. Moreover, even when only one photolithography step isadded in the manufacturing process, the number of steps relating to thephotolithography step is significantly increased. Therefore, manytechniques for reducing the number of photolithography processes havebeen developed.

As typical means for reducing the number of photolithography processesin a manufacturing process of a transistor, a technique using amulti-tone mask (also called a half-tone mask or a gray-tone mask) iswidely known. As examples of the known technique for reducing the numberof manufacturing steps by using a multi-tone mask, Patent Documents 1 to3 are given.

Patent Document 1: Japanese Published Patent Application No. 2012-178545Patent Document 2: Japanese Published Patent Application No. 2011-155303Patent Document 3: Japanese Published Patent Application No. 2009-124124SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to reduce thenumber of masks used in a manufacturing process of a semiconductordevice. Another object is to reduce the number of photolithographyprocesses. Another object is to reduce the manufacturing time of asemiconductor device. Another object is to reduce the manufacturing costof a semiconductor device.

One embodiment of the present invention is a method for manufacturing asemiconductor device that includes the steps of forming a firstconductive film over a substrate; forming a first insulating film overthe first conductive film; forming a semiconductor film over the firstinsulating film; forming a semiconductor film including a channel regionby etching at least part of the semiconductor film; forming a secondinsulating film over the semiconductor film including the channelregion; forming a mask over the second insulating film; performing afirst step in which at the same time as removing a first portion of thesecond insulating film that overlaps the semiconductor film includingthe channel region and that the mask does not overlap, second portionsof the first insulating film and the second insulating film that do notoverlap the semiconductor film including the channel region and that themask does not overlap are removed; removing the mask after the firststep; and forming a second conductive film electrically connected to thesemiconductor film including the channel region over at least part ofthe second insulating film.

It is preferable that in the above-described method, the mask be made torecede to expose part of the second insulating film and the part of thesecond insulating film be removed between the first step and the removalof the mask.

Another embodiment of the present invention is a method formanufacturing a semiconductor device that includes the steps of forminga first conductive film over a substrate; forming a first insulatingfilm over the first conductive film; forming a semiconductor film overthe first insulating film; forming a semiconductor film including achannel region by etching at least part of the semiconductor film;forming a second insulating film over the semiconductor film includingthe channel region; forming a mask including a first region and a secondregion having a thickness smaller than that of the first region over thesecond insulating film; performing a first step of removing portions ofthe first insulating film and the second insulating film that the maskdoes not overlap; performing a second step of removing part of the maskthat is in the second region by making the mask recede after the firststep; performing a third step of removing a portion of the secondinsulating film that the second region overlaps after the second step;removing the mask after the third step; and forming a second conductivefilm electrically connected to the semiconductor film including thechannel region over at least part of the second insulating film.

Another embodiment of the present invention is a method formanufacturing a semiconductor device that includes the steps of forminga first conductive film over a substrate; forming a first insulatingfilm over the first conductive film; forming a semiconductor film overthe first insulating film; forming a mask including a first region and asecond region having a thickness smaller than that of the first regionover the semiconductor film; performing a first step of forming anopening in the first insulating film by removing portions of the firstinsulating film and the semiconductor film that the mask does notoverlap; performing a second step of removing part of the mask that isin the second region by making the mask recede after the first step;performing a third step of forming a semiconductor film including achannel region by removing a portion of the semiconductor film that thesecond region overlaps after the second step; removing the mask afterthe third step; forming a second insulating film over the semiconductorfilm including the channel region; removing at least a portion of thesecond insulating film that overlaps the opening; and forming a secondconductive film electrically connected to the semiconductor filmincluding the channel region over at least part of the second insulatingfilm.

Another embodiment of the present invention is a method formanufacturing a semiconductor device that includes the steps of: forminga first electrode over a substrate; forming a first insulating film overthe first electrode; forming a second insulating film over the firstinsulating film; forming a first conductive film over the secondinsulating film; forming a mask including a first region and a secondregion having a thickness smaller than that of the first region over thefirst conductive film; performing a first step of removing portions ofthe first insulating film, the second insulating film, and the firstconductive film that the mask does not overlap; performing a second stepof removing part of the mask that is in the second region by making themask recede after the first step; performing a third step of removingportions of the first conductive film that the second region overlapsafter the second step; removing the mask after the third step; forming athird insulating film over at least part of the second insulating film;and forming a second conductive film electrically connected to the firstelectrode over at least part of the third insulating film.

In the above-described method, a step of forming a semiconductor filmover the substrate, a step of forming a fourth insulating film over thesemiconductor film, and a step of forming a first opening in the fourthinsulating film may be performed before the formation of the firstelectrode. In the above-described method, the first electrode may beelectrically connected to the semiconductor film through the firstopening formed in the fourth insulating film. In the above-describedmethod, the semiconductor film may include an oxide semiconductor. Inthe above-described method, the first opening may have a tapered shape.In the above-described method, an opening formed in the secondinsulating film by the first step may have a tapered shape. In theabove-described method, an opening formed in the first insulating filmby the first step may have a tapered shape. In the above-describedmethod, the third insulating film may have tapered side surfaces.

Another embodiment of the present invention is a method formanufacturing a semiconductor device that includes the steps of: forminga first electrode over a substrate; forming a first insulating film overthe first electrode; forming a second insulating film over the firstinsulating film; forming a first conductive film over the secondinsulating film; forming a mask including a first region and a secondregion having a thickness smaller than that of the first region over thefirst conductive film; performing a first step of removing portions ofthe first insulating film, the second insulating film, and the firstconductive film that the mask does not overlap; performing a second stepof removing part of the mask that is in the second region by making themask recede after the first step; performing a third step of removingportions of the first conductive film that the second region overlapsafter the second step; removing the mask after the third step; forming athird insulating film over at least part of a top surface of the secondinsulating film and on at least part of a side surface of the secondinsulating film in an opening formed in the second insulating film bythe first step; and forming a second conductive film electricallyconnected to the first electrode over at least part of the thirdinsulating film.

Another embodiment of the present invention is a method formanufacturing a semiconductor device that includes the steps of: forminga first electrode over a substrate; forming a first insulating film overthe first electrode; forming a second insulating film over the firstinsulating film; forming a first conductive film over the secondinsulating film; forming a mask including a first region and a secondregion having a thickness smaller than that of the first region over thefirst conductive film; performing a first step of removing portions ofthe first insulating film, the second insulating film, and the firstconductive film that the mask does not overlap; performing a second stepof removing part of the mask that is in the second region by making themask recede after the first step; performing a third step of removingportions of the first conductive film that the second region overlapsafter the second step; removing the mask after the third step; forming athird insulating film over at least part of a top surface of the secondinsulating film, on a side surface of the second insulating film in anopening formed in the second insulating film by the first step, and overat least part of the first electrode; and forming a second conductivefilm electrically connected to the first electrode over at least part ofthe third insulating film.

According to one embodiment of the present invention, the number ofmasks used in a manufacturing process of a semiconductor device can bereduced. The number of photolithography processes can also be reduced.The manufacturing time of a semiconductor device can also be reduced.The manufacturing cost of a semiconductor device can also be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are top views illustrating a semiconductor device.

FIGS. 2A to 2E are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 3A to 3D are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 4A to 4F are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 5A to 5E are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 6A to 6E are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 7A to 7E are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 8A to 8D are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 9A to 9F are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 10A to 10E are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 11A to 11E are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 12A to 12E are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 13A to 13C illustrate a liquid crystal display device.

FIGS. 14A to 14E illustrate electronic appliances.

FIGS. 15A to 15C are cross-sectional views illustrating a method formanufacturing a semiconductor device that is one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention are described below in detail withreference to the accompanying drawings. Note that the present inventionis not limited to the description below, and it is easily understood bythose skilled in the art that modes and details thereof can be modifiedin various ways. Further, the present invention is not construed asbeing limited to description of the embodiments. In describingstructures of the present invention with reference to the drawings,common reference numerals are used for the same portions in differentdrawings. Note that the same hatched pattern is applied to similarparts, and the similar parts are not especially denoted by referencenumerals in some cases.

Note that the ordinal numbers such as “first” and “second” in thisspecification and the like are used for convenience and do not denotethe order of steps or the stacking order of layers. Therefore, forexample, description can be made even when “first” is replaced with“second” or “third”, as appropriate. In addition, the ordinal numbers inthis specification are not necessarily the same as the ordinal numbersused to specify one embodiment of the present invention.

Note that in this specification and the like, the term such as “over”does not necessarily mean that a component is placed “directly on”another component. For example, the expression “a gate electrode over aninsulating layer” can mean a case where there is an additional componentbetween the insulating layer and the gate electrode. The same applies tothe term “under”. The expression can also mean a case where there is noplanar overlap between the insulating layer and the gate electrode(i.e., the insulating layer and the gate electrode do not overlap eachother).

In addition, in this specification and the like, terms such as“electrode” and “wiring” do not limit the functions of components. Forexample, an “electrode” can be used as part of a “wiring”, and the“wiring” can be used as part of the “electrode”. The terms such as“electrode” and “wiring” can also mean a combination of a plurality of“electrodes” and “wirings”.

Functions of a “source” and a “drain” are sometimes replaced with eachother, for example, when a transistor of opposite polarity is used orwhen the direction of flow of current is changed in circuit operation.Therefore, the terms “source” and “drain” can be replaced with eachother in this specification.

The term “electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on the “object having any electric function” aslong as electric signals can be transmitted and received between thecomponents connected through the object. Examples of the “object havingany electric function” are an electrode and a wiring.

Note that a “semiconductor” can have characteristics of an “insulator”,for example, when the conductivity is sufficiently low. Further, a“semiconductor” and an “insulator” cannot be strictly distinguished fromeach other in some cases because a border between the “semiconductor”and the “insulator” is not clear. Therefore, a “semiconductor” in thisspecification can be called an “insulator” in some cases. Similarly, an“insulator” in this specification can be called a “semiconductor” insome cases.

Further, a “semiconductor” can includes characteristics of a“conductor”, for example, when the conductivity is sufficiently high.Further, a “semiconductor” and a “conductor” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “conductor” is not clear. Therefore, a“semiconductor” in this specification can be called a “conductor” insome cases. Similarly, a “conductor” in this specification can be calleda “semiconductor” in some cases.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. In addition, the term “perpendicular” indicates that theangle formed between two straight lines ranges from 80° to 100°, andaccordingly also includes the case where the angle ranges from 85° to95°.

In this specification and the like, trigonal and rhombohedral crystalsystems are included in a hexagonal crystal system.

Embodiment 1

A method for manufacturing a semiconductor device that is one embodimentof the present invention is described with reference to FIGS. 1A to 1C,FIGS. 2A to 2E, FIGS. 3A to 3D, FIGS. 4A to 4F, FIGS. 5A to 5E, FIGS. 6Ato 6E, FIGS. 7A to 7E, FIGS. 8A to 8D, FIGS. 9A to 9F, and FIGS. 10A to10E. FIGS. 1A to 1C are top views illustrating a semiconductor device.FIG. 1A illustrates a portion that can function as a pixel region of aliquid crystal display device. FIGS. 1B and 1C each illustrate a portionthat can function as a peripheral portion provided with a driver and thelike of the liquid crystal display device. Some components areselectively illustrated for simplification of the drawings; for example,an insulating film 104, an insulating film 108, and the like are notillustrated. FIGS. 2A to 10E each illustrate the cross section X1-X2taken along the dashed dotted line X1-X2 in FIG. 1A. The cross sectionX1-X2 corresponds to a cross-sectional structure of part of a regionwhere a transistor is formed. FIGS. 2A to 10E each also illustrate thecross section Y1-Y2 taken along the dashed dotted line Y1-Y2 in FIG. 1B.The cross section Y1-Y2 corresponds to a cross-sectional structure ofpart of a region where a conductive film under a semiconductor film anda conductive film over the semiconductor film are electrically connectedto each other.

<<Manufacturing Method 1>>

First, a method for forming a transistor 150 and a connection portion160, which are illustrated in FIG. 3D, is described with reference toFIGS. 2A to 2E and FIGS. 3A to 3D.

<Substrate>

First, a substrate 100 is provided (FIG. 2A). There is no particularlimitation on the property of a material and the like of the substrate100 as long as the material has heat resistance high enough to withstandat least heat treatment performed later. For example, a glass substrate,a ceramic substrate, a quartz substrate, a sapphire substrate, or anyttria-stabilized zirconia (YSZ) substrate may be used as the substrate100. Alternatively, a single-crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like can be used as thesubstrate 100.

Alternatively, a semiconductor substrate or an SOI substrate providedwith a semiconductor element may be used as the substrate 100. In thiscase, the transistor 150 is formed over the substrate 100 with aninterlayer insulating layer interposed therebetween. The transistor 150in this case may have a structure in which at least one of a conductivefilm 102 b, a conductive film 114 b, and a conductive film 114 c iselectrically connected to the semiconductor element by a connectionelectrode embedded in the interlayer insulating layer. Forming thetransistor 150 over the semiconductor element with the interlayerinsulating layer interposed therebetween can suppress an increase inarea due to the formation of the transistor 150.

Alternatively, a flexible substrate such as a plastic substrate may beused as the substrate 100, and the transistor 150 may be provideddirectly on the flexible substrate. Further alternatively, a separationlayer may be provided between the substrate 100 and the transistor 150.The separation layer can be used when part or the whole of thetransistor formed over the separation layer is formed and separated fromthe substrate 100 and transferred to another substrate. Thus, thetransistor 150 can be transferred to a substrate having low heatresistance or a flexible substrate.

<Formation of Conductive Film>

Next, a conductive film is formed over the substrate 100 by a sputteringmethod, a CVD method, an evaporation method, or the like, and a resistmask is formed over the conductive film by a photolithography process.After that, part of the conductive film is etched using the resist maskto form a conductive film 102 a and the conductive film 102 b (FIG. 3B).The conductive film 102 b can function as a gate electrode in thetransistor 150.

The conductive film 102 a and the conductive film 102 b can be formedusing a metal selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, and tungsten, an alloy containing any of thesemetals as its component, an alloy containing any of these metals incombination, or the like. Further, one or more metals selected frommanganese and zirconium may be used.

The conductive film 102 a and the conductor film 102 b may have asingle-layer structure or a layered structure of two or more layers. Forexample, a two-layer structure in which a film functioning as a barrierfilm such as a film of a metal selected from tungsten, titanium, andmolybdenum or an alloy containing any of the metals as its component isstacked over or under an aluminum film may be employed. Alternatively, athree-layer structure in which the above-described films functioning asbarrier films are stacked over and under an aluminum film may beemployed. Further alternatively, a two-layer structure in which theabove-described film functioning as a barrier film is stacked over orunder a copper film may be employed. Still alternatively, a three-layerstructure in which the above-described films functioning as barrierfilms are stacked over and under a copper film may be employed.

By using the aluminum film or the copper film, which has low resistance,for the conductive film 102 a and the conductive film 102 b, the powerconsumption of the semiconductor device can be reduced. In addition, bystacking the film functioning as a barrier film, such as a tungstenfilm, a titanium film, or a molybdenum film, to be in contact with thealuminum film or the copper film, diffusion of aluminum or copper can besuppressed and the reliability of the semiconductor device can beimproved.

Further, an In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-basedoxynitride semiconductor film, an In—Ga-based oxynitride semiconductorfilm, an In—Zn-based oxynitride semiconductor film, a Sn-basedoxynitride semiconductor film, an In-based oxynitride semiconductorfilm, or a film of a metal nitride (e.g., InN or ZnN), or the like maybe provided between the conductive film 102 b and the insulating film104 described later. These films each have a work function higher thanor equal 15 to 5 eV, preferably higher than or equal to 5.5 eV, which ishigher than the electron affinity of an oxide semiconductor, thus, thethreshold voltage of a transistor can be shifted in the positivedirection in the case where an oxide semiconductor is used for asemiconductor film described later, and what is called a normally-offswitching element can be obtained. For example, as an In—Ga—Zn-basedoxynitride semiconductor film, an In—Ga—Zn-based oxynitridesemiconductor film having a higher nitrogen concentration than at leasta semiconductor film 106 b, specifically an In—Ga—Zn-based oxynitridesemiconductor film having a nitrogen concentration higher than or equalto 7 atom %, is used.

<Formation of Insulating Film>

Next, an insulating film 104 is formed over the substrate 100, theconductive film 102 a, and the conductive film 102 b (FIG. 2C). Theinsulating film 104 can function as a gate insulating film in thetransistor 150.

The insulating film 104 can be formed to have a single-layer structureor a stacked-layer structure using, for example, one or more of asilicon oxide film, a silicon oxynitride film, a silicon nitride oxidefilm, a silicon nitride film, an aluminum oxide film, a hafnium oxidefilm, a gallium oxide film, and a Ga—Zn-based metal oxide film.

The insulating film 104 may be formed using a high-k material such ashafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(z)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gateleakage current of the transistor can be reduced.

The insulating film 104 is formed by a sputtering method, a CVD method,an evaporation method, or the like.

In the case where a silicon nitride film is formed as the insulatingfilm 104, it is preferable to use a two-step formation method. First, afirst silicon nitride film with a small number of defects is formed by aplasma CVD method in which a mixed gas of silane, nitrogen, and ammoniais used as a source gas. Then, a second silicon nitride film in whichthe hydrogen concentration is low and hydrogen can be blocked is formedby switching the source gas to a mixed gas of silane and nitrogen. Theformation method can form a silicon nitride film with few defects and ablocking property against hydrogen as the insulating film 104.

In the case where a gallium oxide film is formed as the insulating film104, metal organic chemical vapor deposition (MOCVD) method can beemployed.

<Formation of Semiconductor Film>

Next, a semiconductor film is formed over the insulating film 104 by asputtering method, a CVD method, an evaporation method, or the like, anda resist mask is formed over the semiconductor film by aphotolithography process. After that, at least part of the semiconductorfilm is etched using the resist mask to form a semiconductor film 106 aand the semiconductor film 106 b (FIG. 2D). The semiconductor film 106 bincludes a channel region of the transistor 150.

A variety of semiconductor materials can be used for the semiconductorfilm 106 a and the semiconductor film 106 b. Specifically, a singlelayer or a stacked layer of amorphous silicon, microcrystalline silicon,polycrystalline silicon, single-crystal silicon, or the like can beused. In addition, an impurity imparting a conductivity type may beadded to part of the semiconductor film or to at least one layer in thecase of the stacked layer.

Besides the semiconductor materials given above, an oxide semiconductorcan be used. A transistor including an oxide semiconductor has extremelylow off-state current in some cases. By using such a transistor, theholding capability of a signal input to a capacitor of each pixel in aliquid crystal display device can be improved, so that the frame ratein, for example, still-image display can be reduced. The reduction inthe frame rate leads to a reduction in the power consumption of thedisplay device.

An oxide semiconductor that can be used for the semiconductor film 106 aand the semiconductor film 106 b is described below.

The oxide semiconductor contains, for example, indium. An oxidesemiconductor containing indium has high carrier mobility (electronmobility). An oxide semiconductor preferably contains an element M.Examples of the element M include aluminum, gallium, yttrium, and tin.For example, the element M has high bond energy to oxygen. For example,the element M increases the energy gap of the oxide semiconductor.Further, the oxide semiconductor preferably contains zinc. Whencontaining zinc, the oxide semiconductor easily becomes crystalline. Theenergy at the top of the valence band (Ev) of the oxide semiconductorcan be controlled by, for example, the atomic ratio of zinc, in somecases.

The oxide semiconductor does not necessarily contain indium. The oxidesemiconductor may be, for example, a Zn—Sn oxide or a Ga—Sn oxide.

The oxide semiconductor may be an In—M—Zn oxide having any of thefollowing atomic ratios of In to M: the atomic percentage of In issmaller than 50 atomic % and the atomic percentage of M is larger thanor equal to 50 atomic %, and the atomic percentage of In is smaller than25 atomic % and the atomic percentage of M is larger than or equal to 75atomic %, when summation of In and M is assumed to be 100 atomic %.Further, the oxide semiconductor may be an In—M-Zn oxide having any ofthe following atomic ratios of In to M: the atomic percentage of In islarger than or equal to 25 atomic % and the atomic percentage of M issmaller than 75 atomic %, and the atomic percentage of In is larger thanor equal to 34 atomic % and the atomic percentage of M is smaller than66 atomic %, when summation of In and M is assumed to be 100 atomic %.

The oxide semiconductor has a large energy gap. The energy gap of theoxide semiconductor is greater than or equal to 2.7 eV and less than orequal to 4.9 eV, preferably greater than or equal to 3 eV and less thanor equal to 4.7 eV, more preferably greater than or equal to 3.2 eV andless than or equal to 4.4 eV.

In order to obtain stable electrical characteristics of a transistor, itis effective to reduce the concentration of impurities in the oxidesemiconductor so that the oxide semiconductor is highly purified to beintrinsic. In the oxide semiconductor, a light element, a semimetalelement, a metal element, and the like (lower than 1 atomic %) otherthan main components serve as impurities. For example, hydrogen,lithium, carbon, nitrogen, fluorine, sodium, silicon, chlorine,potassium, calcium, titanium, iron, nickel, copper, germanium,strontium, zirconium, and hafnium might be impurities in the oxidesemiconductor. Thus, it is preferable to reduce the concentration ofimpurities in a film adjacent to the oxide semiconductor.

For example, in some cases, silicon in an oxide semiconductor formsimpurity states. Further, in some cases, silicon at the surface of anoxide semiconductor forms impurity states. Thus, the concentration ofsilicon in an oxide semiconductor or at the surface of an oxidesemiconductor, which is measured by secondary ion mass spectrometry(SIMS), is preferably lower than 1×10¹⁹ atoms/cm³, more preferably lowerthan 5×10¹⁸ atoms/cm³, still more preferably lower than 2×10¹⁸atoms/cm³.

Further, in some cases, hydrogen in an oxide semiconductor formsimpurity states, whereby carrier density is increased. Thus, theconcentration of hydrogen in the oxide semiconductor, which is measuredby SIMS, is lower than or equal to 2×10²⁰ atoms/cm³, preferably lowerthan or equal to 5×10¹⁹ atoms/cm³, more preferably lower than or equalto 1×10¹⁹ atoms/cm³, still more preferably lower than or equal to 5×10¹⁸atoms/cm³. Further, in some cases, nitrogen in an oxide semiconductorforms impurity levels, whereby carrier density is increased. Thus, theconcentration of nitrogen in the oxide semiconductor, which is measuredby SIMS, is lower than 5×10¹⁹ atoms/cm³, preferably lower than or equalto 5×10¹⁸ atoms/cm³, more preferably lower than or equal to 1×10¹⁸atoms/cm³, still more preferably lower than or equal to 5×10¹⁷atoms/cm³.

An oxide semiconductor film is classified roughly into a single-crystaloxide semiconductor film and a non-single-crystal oxide semiconductorfilm. The non-single-crystal oxide semiconductor film includes any of ac-axis aligned crystalline oxide semiconductor (CAAC-OS) film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and the like.

An oxide semiconductor film may include a CAAC-OS film. First, theCAAC-OS film is described.

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fitinside a cube whose one side is less than 100 nm. Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in the directionsubstantially parallel to a sample surface (cross-sectional TEM image),metal atoms are arranged in a layered manner in the crystal parts. Eachmetal atom layer has a morphology reflected by a surface over which theCAAC-OS film is formed (hereinafter, a surface over which the CAAC-OSfilm is formed is referred to as a formation surface) or a top surfaceof the CAAC-OS film, and is arranged in parallel to the formationsurface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 310.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in the direction substantiallyperpendicular to the c-axis, a peak appears frequently when 2θ is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when 0 scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallel toa normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel to the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned inthe direction parallel to a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where theshape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel to a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

The degree of crystallinity in the CAAC-OS film is not necessarilyuniform. For example, in the case where crystal growth leading to theCAAC-OS film occurs from the vicinity of the top surface of the film,the degree of the crystallinity in the vicinity of the top surface ishigher than that in the vicinity of the formation surface in some cases.Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

The CAAC-OS film is an oxide semiconductor film having low impurityconcentration. The impurity is an element other than the main componentsof the oxide semiconductor film, such as hydrogen, carbon, silicon, or atransition metal element. In particular, an element that has higherbonding strength to oxygen than a metal element included in the oxidesemiconductor film, such as silicon, disturbs the atomic arrangement ofthe oxide semiconductor film by depriving the oxide semiconductor filmof oxygen and causes a decrease in crystallinity. Further, a heavy metalsuch as iron or nickel, argon, carbon dioxide, or the like has a largeatomic radius (molecular radius), and thus disturbs the atomicarrangement of the oxide semiconductor film and causes a decrease incrystallinity when it is contained in the oxide semiconductor film. Notethat the impurity contained in the oxide semiconductor film might serveas a carrier trap or a carrier generation source.

The CAAC-OS film is an oxide semiconductor film having a low density ofdefect states. For example, oxygen vacancies in the oxide semiconductorfilm serve as carrier traps or serve as carrier generation sources insome cases when hydrogen is captured therein.

The state in which the impurity concentration is low and the density ofdefect states is low (the number of oxygen vacancies is small) isreferred to as a highly purified intrinsic state or a substantiallyhighly purified intrinsic state. A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has fewcarrier generation sources, and thus can have a low carrier density.Thus, a transistor including the oxide semiconductor film rarely hasnegative threshold voltage (is rarely normally on). The highly purifiedintrinsic or substantially highly purified intrinsic oxide semiconductorfilm has few carrier traps. Accordingly, the transistor including theoxide semiconductor film has small variations in electricalcharacteristics and high reliability. Electric charge trapped by thecarrier traps in the oxide semiconductor film takes a long time to bereleased, and might behave like fixed electric charge. Thus, thetransistor that includes the oxide semiconductor film having highimpurity concentration and a high density of defect states has unstableelectrical characteristics in some cases.

With the use of the CAAC-OS film in a transistor, variation in theelectrical characteristics of the transistor due to irradiation withvisible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor film is described.

In an image obtained with the TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, the size of a crystal part in the microcrystalline oxidesemiconductor is greater than or equal to 1 nm and less than or equal to100 nm, or greater than or equal to 1 nm and less than or equal to 10nm. A microcrystal with a size greater than or equal to 1 nm and lessthan or equal to 10 nm, or a size greater than or equal to 1 nm and lessthan or equal to 3 nm is specifically referred to as nanocrystal (nc).An oxide semiconductor film including nanocrystal is referred to as annc-OS (nanocrystalline oxide semiconductor) film. In an image of thenc-OS obtained with a TEM, for example, a crystal grain cannot beobserved clearly in some cases.

In the nc-OS film, a microscopic region (for example, a region with asize greater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic order. Further, there is noregularity of crystal orientation between different crystal parts in thenc-OS film; thus, the orientation is not observed in the whole film.Accordingly, in some cases, the nc-OS film cannot be distinguished froman amorphous oxide semiconductor depending on an analysis method. Forexample, when the nc-OS film is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than that of a crystal part, a peak which shows acrystal plane does not appear. Further, a halo pattern is shown in anelectron diffraction pattern (also referred to as a selected-areaelectron diffraction area) of the nc-OS film obtained by using anelectron beam having a diameter (e.g., larger than or equal to 50 nm)larger than that of a crystal part. Meanwhile, spots are shown in ananobeam electron diffraction pattern of the nc-OS film obtained byusing an electron beam having a diameter (e.g., larger than or equal to1 nm and smaller than or equal to 30 nm) close to, or smaller than orequal to that of a crystal part. Further, in a nanobeam electrondiffraction pattern of the nc-OS film, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS film, a plurality of spots areshown in a ring-like region in some cases.

Since the nc-OS film is an oxide semiconductor film having moreregularity than the amorphous oxide semiconductor film, the nc-OS filmhas a lower density of defect states than the amorphous oxidesemiconductor film. However, there is no regularity of crystalorientation between different crystal parts in the nc-OS film; hence,the nc-OS film has a higher density of defect states than the CAAC-OSfilm.

Note that an oxide semiconductor film may be a stacked film includingtwo or more kinds of, for example, an amorphous oxide semiconductorfilm, a microcrystalline oxide semiconductor film, and a CAAC-OS film.

For example, a multilayer film in which an oxide semiconductor layer(S1) and an oxide semiconductor layer (S2) are formed in this order maybe used.

In this case, for example, the energy (Ec) at the bottom of theconduction band of the oxide semiconductor layer (S2) is set to behigher than that of the oxide semiconductor layer (S1). Specifically,for the oxide semiconductor layer (S2), an oxide semiconductor havinglower electron affinity than the oxide semiconductor layer (S1) bygreater than or equal to 0.07 eV and less than or equal to 1.3 eV,preferably greater than or equal to 0.1 eV and less than or equal to 0.7eV, more preferably greater than or equal to 0.15 eV and less than orequal to 0.4 eV is used. Note that the electron affinity refers to anenergy gap between the vacuum level and the bottom of the conductionband.

Alternatively, for example, the energy gap of the oxide semiconductorlayer (S2) is set to be larger than that of the oxide semiconductorlayer (S1). The energy gap can be obtained by, for example, an opticalmethod. Specifically, for the oxide semiconductor layer (S2), an oxidesemiconductor having a larger energy gap than the oxide semiconductorlayer (S1) by greater than or equal to 0.1 eV and less than or equal to1.2 eV, preferably greater than or equal to 0.2 eV and less than orequal to 0.8 eV is used.

Alternatively, the oxide semiconductor may be, for example, a multilayerfilm in which the oxide semiconductor layer (S1), the oxidesemiconductor layer (S2), and an oxide semiconductor layer (S3) areformed in this order.

For example, the energy (Ec) at the bottom of the conduction band of theoxide semiconductor layer (S2) is set to be lower than that of the oxidesemiconductor layer (S1) and the oxide semiconductor layer (S3).Specifically, for the oxide semiconductor layer (S2), an oxidesemiconductor having higher electron affinity than the oxidesemiconductor layer (S1) and the oxide semiconductor film (S3) bygreater than or equal to 0.07 eV and less than or equal to 1.3 eV,preferably greater than or equal to 0.1 eV and less than or equal to 0.7eV, more preferably greater than or equal to 0.15 eV and less than orequal to 0.4 eV is used.

Alternatively, for example, the energy gap of the oxide semiconductorlayer (S2) may be smaller than that of each of the oxide semiconductorlayers (S1) and (S3). Specifically, for the oxide semiconductor layer(S2), an oxide semiconductor having smaller energy gap than the oxidesemiconductor layers (S1) and (S3) by greater than or equal to 0.1 eVand smaller than or equal to 1.2 eV or by greater than or equal to 0.2eV and smaller than or equal to 0.8 eV is used.

For example, to increase the on-state current of the transistor, thethickness of the oxide semiconductor layer (S3) is preferably as low aspossible. For example, the thickness of the oxide semiconductor layer(S3) is less than 10 nm, preferably less than or equal to 5 nm, morepreferably less than or equal to 3 nm. Meanwhile, the oxidesemiconductor layer (S3) blocks entry of elements (e.g., silicon)contained in the insulating film 104 to the oxide semiconductor layer(S2) having a high current density. Thus, the oxide semiconductor layer(S3) preferably has a certain thickness. The thickness of the oxidesemiconductor layer (S3) is, for example, greater than or equal to 0.3nm, preferably greater than or equal to 1 nm, more preferably greaterthan or equal to 2 nm.

The thickness of the oxide semiconductor layer (S1) is preferably largerthan that of the oxide semiconductor layer (S2), and the thickness ofthe oxide semiconductor layer (S2) may be larger than that of the oxidesemiconductor layer (S3). Specifically, the thickness of the oxidesemiconductor layer (S1) is greater than or equal to 20 nm, preferablygreater than or equal to 30 nm, more preferably greater than or equal to40 nm, still more preferably greater than or equal to 60 nm. With theoxide semiconductor layer (S1) having the above thickness, the interfacebetween the insulating film and the oxide semiconductor layer (S1) canbe separated from the oxide semiconductor layer (S2) with high currentdensity to have a distance greater than or equal to 20 nm, preferablygreater than or equal to 30 nm, more preferably greater than or equal to40 nm, still more preferably greater than or equal to 60 nm. To preventthe productivity of the semiconductor device from being lowered, thethickness of the oxide semiconductor layer (S1) is less than or equal to200 nm, preferably less than or equal to 120 nm, more preferably lessthan or equal to 80 nm. The thickness of the oxide semiconductor layer(S2) is greater than or equal to 3 nm and less than or equal to 100 nm,preferably greater than or equal to 3 nm and less than or equal to 80nm, more preferably greater than or equal to 3 nm and less than or equalto 50 nm.

Next, a method for forming the oxide semiconductor film is described.The oxide semiconductor film may be formed by a sputtering method, a CVDmethod, an MBE method, an ALD method, or a PLD method.

When In—M-Zn oxide is formed by a sputtering method for the oxidesemiconductor film to be a semiconductor film 106, the atomic ratio ofthe target may be as follows: In:M:Z=3:1:1, 3:1:2, 3:1:4, 1:1:0.5,1:1:1, 1:1:2, 1:3:1, 1:3:2, 1:3:4, 1:3:6, 1:6:2, 1:6:4, 1:6:6, 1:6:8,1:6:10, 1:9:2, 1:9:4, 1:9:6, 1:9:8, 1:9:10, or the like. Examples of theelement M include aluminum, gallium, yttrium, and tin.

When the oxide semiconductor film is formed by a sputtering method, itis formed under an atmosphere containing oxygen. The proportion ofoxygen in the atmosphere is, for example, larger than or equal to 10volume %, preferably larger than or equal to 20 volume %, morepreferably larger than or equal to 50 volume %, still more preferablylarger than or equal to 80 volume %. In particular, the proportion ofoxygen in the atmosphere is preferably 100 volume %. When the proportionof oxygen in the atmosphere is 100 volume %, the concentration ofimpurities (e.g., a rare gas) contained in the oxide semiconductor filmto be the semiconductor film 106 can be reduced.

In the case where the oxide semiconductor film to be the semiconductorfilm 106 is formed by a sputtering method, a film having an atomic ratiodeviated from the atomic ratio of the target is formed in some cases.For example, when zinc is formed under an atmosphere containing oxygen,the atomic ratio of zinc in the formed film is easily smaller than thatin the target in some cases. Specifically, the atomic percentage of zincin the film is higher than or equal to approximately 40% and lower thanor equal to approximately 90% of that of zinc in the target in somecases. Further, for example, when indium is formed under an atmospherecontaining oxygen, the atomic ratio of indium in the formed film iseasily smaller than that in the target in some cases.

First heat treatment is preferably performed after the oxidesemiconductor film to be the semiconductor film 106 is formed. The firstheat treatment may be performed at higher than or equal to 70° C. andlower than or equal to 450° C., preferably higher than or equal to 100°C. and lower than or equal to 300° C., more preferably higher than orequal to 150° C. and lower than or equal to 250° C. The first heattreatment is performed in an inert gas atmosphere or an atmospherecontaining an oxidizing gas at 10 ppm or more, 1 volume % or more, or 10volume % or more. The first heat treatment may be performed under areduced pressure. Alternatively, the first heat treatment may beperformed in such a manner that heat treatment is performed in an inertgas atmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, 1 volume % ormore, or 10 volume % or more in order to compensate desorbed oxygen. Thefirst heat treatment can remove impurities such as hydrogen and waterfrom the oxide semiconductor film to be the oxide semiconductor film106. In addition, the first heat treatment can highly purify the oxidesemiconductor film to be the oxide semiconductor film 106.

<Formation of Insulating Film>

Next, an insulating film 108 is formed over the insulating film 104, thesemiconductor film 106 a, and the semiconductor film 106 b (FIG. 2E).The insulating film 108 functions as a film that protects the channelregion in the semiconductor film 106 b in the transistor 150.

The insulating film 108 can be formed using a material, a structure, anda method similar to those of the insulating film 104.

In the case where the insulating film 104 has a layered structure offilms formed of different materials, the same material can be used foran upper layer of the insulating film 104 and the insulating film 108.When the same material is used for the upper layer of the insulatingfilm 104 and the insulating film 108, the upper layer of the insulatingfilm 104 and the insulating film 108 can be etched at the same time inetching of the insulating film 104 and the insulating film 108 in alater step. After that, a remaining lower layer of the insulating film104 is etched. Thus, an opening in which the taper angle of the upperlayer of the insulating film 104 and the insulating film 108 isdifferent from the taper angle of the lower layer of the insulating film104 can be formed. The opening having the different taper angles canprevent disconnection of a conductive film formed in the opening.

<Formation of Mask and Etching of Insulating Film>

Next, a resist mask 110 is formed over the insulating film 108 by aphotolithography process (FIG. 3A).

Next, portions of the insulating film 104 and the insulating film 108that the resist mask 110 does not overlap are removed by etching to forman opening 111 a, an opening 111 b, an opening 111 c, and an opening 111d (FIG. 3B).

As illustrated in FIG. 3B, the openings 111 a, 111 c, and 111 d areformed in such a manner that portions of the insulating film 108 thatoverlap the semiconductor film 106 a and 106 b and that the resist mask110 does not overlap are removed.

The opening 111 b is formed in such a manner that portions of theinsulating film 104 and the insulating film 108 that overlaps theconductive film 102 a and overlaps neither the semiconductor film 106 anor the semiconductor film 106 b and that the resist mask 110 does notoverlap are removed.

Then, the resist mask 110 is removed (see FIG. 3C).

<Formation of Conductive Film>

Next, a conductive film is formed over the insulating film 108, thesemiconductor film 106 a, the semiconductor film 106 b, and theconductive film 102 a, and a resist mask is formed over the conductivefilm by a photolithography process. After that, part of the conductivefilm is etched using the resist mask to form a conductive film 114 a, aconductive film 114 b, and a conductive film 114 c (FIG. 3D).

The conductive film 114 a is electrically connected to the conductivefilm 102 a and the semiconductor film 106 a. In addition, the conductivefilm 114 b and the conductive film 114 c are electrically connected tothe semiconductor film 106 b.

The conductive film 114 a can function as a wiring that electricallyconnects the conductive film 102 a and the semiconductor film 106 a. Theconductive film 114 b can function as a source electrode in thetransistor 150, and the conductive film 114 c can function as a drainelectrode in the transistor 150.

The conductive films 114 a, 114 b, and 114 c can be formed using amaterial, a structure, and a method similar to those of the conductivefilms 102 a and 102 b.

Through the above-described steps, the transistor 150 and the connectionportion 160 can be formed.

As described above, in the manufacturing method 1, at the same time asforming the openings in the portions of the insulating film 108 thatoverlap the semiconductor films, the opening is formed in the insulatingfilm 104 as well as in the insulating film 108 in the portion that doesnot overlap the semiconductor films. This step enables the number ofmasks and the number of photolithography processes to be reducedcompared to the case where openings are formed separately in theinsulating film 108 and the insulating film 104. Thus, the manufacturingtime and manufacturing cost of the semiconductor device can be reduced.

<<Manufacturing Method 2>>

A method for forming the transistor 150 and the connection portion 160,which are illustrated in FIG. 5E, is described with reference to FIGS.4A to 4F and FIGS. 5A to 5E.

The substrate 100, the conductive films 102 a and 102 b, the insulatingfilm 104, the semiconductor films 106 a and 106 b, and the insulatingfilm 108, which are illustrated in FIGS. 4A to 4E, can be formed usingmaterials, structures, and methods similar to those in the manufacturingmethod 1.

<Formation of Mask and Etching of Insulating Film>

A resist mask 210 is formed over the insulating film 108 by aphotolithography process (FIG. 4F). As illustrated in FIG. 4F, theresist mask 210 includes regions with different thicknesses: a region210 a and a region 210 b with a thickness smaller than that of theregion 210 a. At least part of the region 210 b overlaps thesemiconductor film 106 a and the semiconductor film 106 b.

The resist mask 210 is formed by a photolithography process using amulti-tone mask (a half-tone photomask or a gray-tone photomask).

Next, portions of the insulating film 104 and the insulating film 108that overlap the conductive film 102 a and that the resist mask 210 doesnot overlap are removed by etching to form the opening 111 b (FIG. 5A).

Next, ashing is performed on the resist mask 210. The ashing reduces thearea (the volume in three dimensions) and thickness of the resist mask210. Thus, part of the resist mask 210 that is in the region 210 b andhas a small thickness is removed to form a resist mask 212 (FIG. 5B). Inother words, the part of the resist mask 210 that is in the region 210 bis removed by making the resist mask 210 recede, whereby the resist mask212 is formed.

The ashing can be performed with, for example, oxygen plasma.

Next, portions of the insulating film 108 that the resist mask 212 doesnot overlap are removed by etching to form an opening 211 a, an opening211 b, an opening 211 c, and an opening 211 d (FIG. 5C).

As illustrated in FIG. 5C, the openings 211 a, 211 c, and 211 d areformed in such a manner that portions of the insulating film 108 thatoverlap the semiconductor films 106 a and 106 b and that the resist mask212 does not overlap are removed.

The opening 211 b is formed in such a manner that a portion of theinsulating film 108 that overlaps neither the semiconductor film 106 anor the semiconductor film 106 b and that the resist mask 212 does notoverlap is removed. As illustrated in FIG. 5C, the opening 211 b isformed in such a manner that part of the insulating film 108 that is inthe vicinity of the opening 111 b is removed to enlarge the opening inthe insulating film 108. This can prevent disconnection of a conductivefilm to be formed over and in the opening 211 b in a later step.

Then, the resist mask 212 is removed (see FIG. 5D).

<Formation of Conductive Film>

Next, a conductive film is formed over the insulating film 108, thesemiconductor film 106 a, the semiconductor film 106 b, and theconductive film 102 a, and a resist mask is formed over the conductivefilm by a photolithography process. After that, part of the conductivefilm is etched using the resist mask to form the conductive films 114 a,114 b, and 114 c (FIG. 5E).

The conductive films 114 a, 114 b, and 114 c can be formed using amaterial, a structure, and a method similar to those in themanufacturing method 1.

Through the above-described steps, the transistor 150 and the connectionportion 160 can be formed.

In the manufacturing method 2, as described above, the use of themulti-tone mask enables the openings 211 a, 211 b, 211 c, and 211 d tobe formed using one mask. In addition, in the opening 211 b, an openingin the insulating film 108 can be larger than an opening in theinsulating film 104, which can prevent disconnection of the conductivefilm 114 a.

The above-described steps can reduce the number of masks and the numberof photolithography processes. Thus, the manufacturing time andmanufacturing cost of the semiconductor device can be reduced. Inaddition, the yield and reliability of the semiconductor device can beimproved.

<<Manufacturing Method 3>>

A method for forming the transistor 150 and the connection portion 160,which are illustrated in FIG. 7E, is described with reference to FIGS.6A to 6E and FIGS. 7A to 7E.

The substrate 100, the conductive films 102 a and 102 b, and theinsulating film 104, which are illustrated in FIGS. 6A to 6C, can beformed using materials, structures, and methods similar to those in themanufacturing method 1.

<Formation of Semiconductor Film>

A semiconductor film 106 is formed over the insulating film 104. Thesemiconductor film 106 can be formed using a material, a structure, anda method similar to those in the manufacturing method 1.

<Formation of Mask and Etching of Insulating Film and SemiconductorFilm>

A resist mask 307 is formed over the semiconductor film 106 by aphotolithography process (FIG. 6D). As illustrated in FIG. 6D, theresist mask 307 includes regions with different thicknesses: a region307 a and a region 307 b with a thickness smaller than that of theregion 307 a. At least part of the region 3076 overlaps thesemiconductor film 106.

The resist mask 307 is formed by a photolithography process using amulti-tone mask (a half-tone photomask or a gray-tone photomask).

Next, portions of the insulating film 104 and the semiconductor film 106that overlap the conductive film 102 a and that the resist mask 307 doesnot overlap are removed by etching to form an opening 308 b (FIG. 6E).

Next, ashing is performed on the resist mask 307. The ashing reduces thearea (the volume in three dimensions) of the resist mask 307. Thus, partof the resist mask 307 that is in the region 307 b and has a smallthickness is removed to form a resist mask 309 (FIG. 7A). In otherwords, the part of the resist mask 307 that is in the region 307 b isremoved by making the resist mask 307 recede, whereby the resist mask309 is formed.

The ashing can be performed with, for example, oxygen plasma.

Next, a portion of the semiconductor film 106 that the resist mask 309does not overlap is removed by etching to form the semiconductor film106 a and the semiconductor film 106 b (FIG. 7B). The semiconductor film106 b can function as a semiconductor film including a channel region ofthe transistor 150.

Then, the resist mask 309 is removed (FIG. 7C).

<Formation of Insulating Film>

Next, an insulating film is formed over the insulating film 104, thesemiconductor film 106 a, and the semiconductor film 106 b, and a resistmask is formed over the insulating film by a photolithography process.Then, part of the insulating film is etched using the resist mask toform the insulating film 108 that has an opening 311 a, an opening 311b, an opening 311 c, and an opening 311 d (FIG. 7D).

As illustrated in FIG. 7D, the openings 311 a, 311 c, and 311 d areformed in portions that overlap the semiconductor film 106 a and thesemiconductor film 106 b.

The opening 311 b is formed in such a manner that a portion of theinsulating film that overlaps neither the semiconductor film 106 a northe semiconductor film 106 b is removed. Note that as illustrated inFIG. 7D, the opening 311 b is formed in a portion overlapping theopening 308 b. In addition, in the opening 311 b, an opening in theinsulating film 108 can be larger than an opening in the insulating film104. This can prevent disconnection of a conductive film to be formedover and in the opening 311 b in a later step.

The insulating film 108 functions as a film that protects the channelregion in the semiconductor film 106 b in the transistor 150 and can beformed using a material, a structure, and a method similar to those inthe manufacturing method 1.

<Formation of Conductive Film>

Next, a conductive film is formed over the insulating film 108, thesemiconductor film 106 a, the semiconductor film 106 b, and theconductive film 102 a, and a resist mask is formed over the conductivefilm by a photolithography process. After that, part of the conductivefilm is etched using the resist mask to form the conductive films 114 a,114 b, and 114 c (FIG. 7E).

The conductive films 114 a, 114 b, and 114 c can be formed using amaterial, a structure, and a method similar to those in themanufacturing method 1.

Through the above-described steps, the transistor 150 and the connectionportion 160 can be formed.

In the manufacturing method 3, as described above, the use of thehalf-tone photomask enables the opening in the insulating film 104 andthe semiconductor films 106 a and 106 b to be formed using one mask. Inaddition, in the opening 311 b, the opening in the insulating film 108can be larger than the opening in the insulating film 104, which canprevent disconnection of the conductive film 114 a.

The above-described steps can reduce the number of masks and the numberof photolithography processes. Thus, the manufacturing time andmanufacturing cost of the semiconductor device can be reduced. Inaddition, the yield and reliability of the semiconductor device can beimproved.

<Variations on Opening>

Note that the connection portion 160 illustrated in FIG. 8D, which has adifferent shape from the connection portion 160 illustrated in FIG. 7E,can be formed by changing the shape of the resist mask 307.

First, the resist mask 307 that has a shape illustrated in FIG. 8A(e.g., a shape in which an opening larger than the opening illustratedin FIG. 6E is provided) is formed to form the opening 308 b (FIG. 8A).

After that, the semiconductor film 106 a and the semiconductor film 106b are formed by steps similar to those illustrated in FIGS. 7A to 7C(FIG. 81B).

Next, an insulating film is formed over the insulating film 104, thesemiconductor film 106 a, and the semiconductor film 106 b, and a resistmask is formed over the insulating film by a photolithography process.Then, part of the insulating film is etched using the resist mask toform the insulating film 108 that has the openings 311 a, 311 c, and 311d and an opening 312 b. In this case, the opening 312 b in theinsulating film 108 is formed so as to be smaller than the opening 308 bin the insulating film 104.

Through the above-described steps, the connection portion 160illustrated in FIG. 8D can be formed.

Even the above-described steps enable the opening in the insulating film104 and the semiconductor films 106 a and 106 b to be formed using onemask and disconnection of the conductive film 114 a to be prevented.Thus, the number of masks, the number of photolithography processes, andthe manufacturing time and manufacturing cost of the semiconductordevice can be reduced. In addition, the yield and reliability of thesemiconductor device can be improved.

<<Manufacturing Method 4>>

A method for forming the transistor 150 and the connection portion 160,which are illustrated in FIG. 10E, is described with reference to FIGS.9A to 9F and FIGS. 10A to 10E.

The substrate 100, the conductive films 102 a and 102 b, the insulatingfilm 104, the semiconductor films 106 a and 106 b, the insulating film108, and the openings 111 a, 111 b, 111 c, and 111 d, which areillustrated in FIGS. 9A to 9F and FIG. 10A, can be formed usingmaterials, structures, and methods similar to those in the manufacturingmethod 1.

<Ashing of Mask>

The openings 111 a, 111 b, 111 c, and 111 d are formed using the resistmask 110 as illustrated in FIG. 10A, and then ashing is performed on theresist mask 110. The ashing can be performed with, for example, oxygenplasma.

The ashing reduces the area (the volume in three dimensions) andthickness of the resist mask 110 to form a resist mask 412 (FIG. 10B).As a result, parts of the insulating film 108 that the resist mask 412does not overlap are formed. In other words, parts of the insulatingfilm 108 are exposed by making the resist mask 110 recede.

Next, the portions of the insulating film 108 that the resist mask 412does not overlap are removed by etching to form an opening 411 a, anopening 411 b, an opening 411 c, and an opening 411 d (FIG. 10C).

As illustrated in FIG. 10C, the openings 411 a, 411 c, and 411 d areformed in portions that overlap the semiconductor film 106 a and thesemiconductor film 106 b.

The opening 411 b is formed in such a manner that a portion of theinsulating film that overlaps the conductive film 102 a and overlapsneither the semiconductor film 106 a nor the semiconductor film 106 b isremoved. As illustrated in FIG. 10C, the opening 411 b is formed in sucha manner that part of the insulating film 108 that is in the vicinity ofthe opening 111 b is removed to enlarge the opening in the insulatingfilm 108. This can prevent disconnection of a conductive film to beformed over and in the opening 411 b in a later step.

The resist mask 412 is formed by ashing the resist mask 110. Thus, thedifferences in size between the resist mask 110 and the resist mask 412,that is, the widths of parts of the insulating film 108 that are notcovered with the resist mask 412 are substantially equal.

Thus, the widths of steps formed by the insulating film 104 and theinsulating film 108 in the opening 411 b, that is, Ls in FIG. 10C aresubstantially equal in the entire periphery of the opening 411 b.

Then, the resist mask 412 is removed (see FIG. 10D).

<Formation of Conductive Film>

Next, a conductive film is formed over the insulating film 108, thesemiconductor film 106 a, the semiconductor film 106 b, and theconductive film 102 a, and a resist mask is formed over the conductivefilm by a photolithography process. After that, part of the conductivefilm is etched using the resist mask to form the conductive films 114 a,114 b, and 114 c (FIG. 10E).

The conductive films 114 a, 114 b, and 114 c can be formed using amaterial, a structure, and a method similar to those in themanufacturing method 1.

Through the above-described steps, the transistor 150 and the connectionportion 160 can be formed.

As described above, in the manufacturing method 4, at the same time asforming the openings in the portions of the insulating film 108 thatoverlap the semiconductor films, the opening is formed in the insulatingfilm 104 as well as in the insulating film 108 in the portion that doesnot overlap the semiconductor films. This step enables the number ofmasks and the number of photolithography processes to be reducedcompared to the case where openings are formed separately in theinsulating film 108 and the insulating film 104. In addition, the ashingperformed on the resist mask 110 enables the opening in the insulatingfilm 108 to be larger than the opening in the insulating film 104 in theopening 411 b, which can prevent disconnection of the conductive film114 a.

The above-described steps can reduce the number of masks and the numberof photolithography processes. Thus, the manufacturing time andmanufacturing cost of the semiconductor device can be reduced. Inaddition, the yield and reliability of the semiconductor device can beimproved.

Embodiment 2

A method for manufacturing a semiconductor device that is one embodimentof the present invention is described with reference to FIGS. 1A to 1C,FIGS. 1A to 11E, and FIGS. 12A to 12E. FIGS. 1A to 1C are top viewsillustrating the semiconductor device as described above. FIGS. 11A to12E each illustrate the cross section X1-X2 taken along the dasheddotted line X1-X2 in FIG. 1A. The cross section X1-X2 corresponds to across-sectional structure of part of a region where a transistor and aconductive film provided over the transistor are electrically connectedto each other. FIGS. 11A to 12E each also illustrate the cross sectionY1-Y2 taken along the dashed dotted line Y1-Y2 in FIG. 1B. The crosssection Y1-Y2 corresponds to a cross-sectional structure of part of aregion where a conductive film provided under a semiconductor film and aconductive film provided over the semiconductor film are electricallyconnected to each other. FIGS. 11A to 12E each also illustrate the crosssection Z1-Z2 taken along the dashed dotted line Z1-Z2 in FIG. 1C. Thecross section Z1-Z2 corresponds to a cross-sectional structure of partof a region where a conductive film formed at the same time asconductive films that can function as a source electrode and a drainelectrode of the transistor is electrically connected to two conductivefilms formed above the transistor.

The method for manufacturing a semiconductor device that is described inthis embodiment can be applied to a case where a plurality of conductivefilms are formed above a transistor. For example, the method is suitablefor a liquid crystal display device in an in-plane-switching (IPS) modeor a fringe field switching (FFS) mode where a common electrode and apixel electrode are formed on the same substrate.

<<Manufacturing Method 5>>

A method for forming a connection portion 550 and a connection portion560, which are illustrated in FIG. 12E, is described with reference toFIGS. 11A to 11E and FIGS. 12A to 12E.

<Lower Structure>

First, a substrate provided with a conductive film is prepared. Thesubstrate may be the substrate over which the transistor 150 and theconnection portion 160 are formed that is described in Embodiment 1 or asubstrate over which a conductive film having a different structure isformed by another method. The substrate is not limited to, for example,a substrate provided with the bottom-gate transistor 150 described inEmbodiment 1 and may be a substrate provided with a top-gate transistor.

In this embodiment, a substrate provided with the transistor 150 in thecross section X1-X2 and the connection portion 160 in the cross sectionY1-Y2, which are formed in a manner similar to that of the manufacturingmethod 2 in Embodiment 1, and the insulating film 104, the insulatingfilm 108, and the conductive film 114 d in the cross section Z1-Z2,which are formed through steps similar to those in the manufacturingmethod 2, is prepared (FIG. 11A). The conductive film 114 d can beformed using a material, a structure, and a method similar to those ofthe conductive films 114 a, 114 b, and 114 c. The connection portion 550and the connection portion 560 are formed over the conductive film 114 aand the conductive film 114 d of the substrate through steps describedbelow.

<Formation of Insulating Film>

An insulating film 500 is formed over the conductive films 114 a, 114 b,114 c, and 114 d (FIG. 11B). The insulating film 500 can be formed usinga material, a structure, and a method similar to those of the insulatingfilm 104 in Embodiment 1.

<Formation of Insulating Film>

Next, an insulating film 502 is formed over the insulating film 500(FIG. 11C). The insulating film 502 can function as a planarization filmin the semiconductor device.

The insulating film 502 can be formed of a single layer or a stackedlayer of for example, acrylic, acrylamide, ester, and another knownmaterial.

<Formation of Conductive Film>

Next, a conductive film 504 is formed over the insulating film 502 (FIG.11D). A light-transmitting material is preferably used for theconductive film 504.

For the conductive film 504, a light-transmitting conductive materialsuch as indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide(hereinafter referred to as ITO), indium zinc oxide, or indium tin oxideto which silicon oxide is added can be used.

A conductive composition containing a conductive macromolecule (alsoreferred to as a conductive polymer) can be used to form the conductivefilm 504. The sheet resistance of the light-transmitting conductive filmformed using a conductive composition is preferably less than or equalto 10000 Ω/square and light transmittance thereof is preferably greaterthan or equal to 70% at a wavelength of 550 nm. Further, the resistivityof the conductive high molecule contained in the conductive compositionis preferably 0.1 Ω·cm or less.

As the conductive high molecule, what is called a π-electron conjugatedconductive polymer can be used. Examples are polyaniline and aderivative thereof, polypyrrole and a derivative thereof, polythiopheneand a derivative thereof, and a copolymer of two or more of aniline,pyrrole, and thiophene and a derivative thereof.

The conductive film 504 can be formed of a single layer or a stackedlayer of any of the materials given above.

<Formation of Mask and Etching of Conductive Film and Insulating Film>

Next, a resist mask 506 is formed over the conductive film 504 by aphotolithography process (FIG. 11E). As illustrated in FIG. 11E, theresist mask 506 includes regions with different thicknesses: a region506 a and a region 506 b with a thickness smaller than that of theregion 506 a. At least part of the region 506 b overlaps the conductivefilm 504.

The resist mask 506 is formed by a photolithography process using amulti-tone mask (a half-tone photomask or a gray-tone photomask).

Next, portions of the insulating film 500, the insulating film 502, andthe conductive film 504 that the resist mask 506 does not overlap areremoved by etching to form an opening 507 a and an opening 507 c (FIG.12A).

Next, ashing is performed on the resist mask 506. The ashing reduces thearea (the volume in three dimensions) and thickness of the resist mask506. Thus, part of the resist mask 506 that is in the region 506 b andhas a small thickness is removed to form a resist mask 508 (FIG. 12B).In other words, the part of the resist mask 506 that is in the region506 b is removed by making the resist mask 506 recede, whereby theresist mask 508 is formed.

The ashing can be performed with, for example, oxygen plasma.

Next, a portion of the conductive film 504 that the resist mask 508 doesnot overlap is removed by etching, and then the resist mask 508 isremoved to form a conductive film 504 a (FIG. 12C). The conductive film504 a can function as a common electrode in a liquid crystal displaydevice.

<Formation of Insulating Film>

Next, an insulating film is formed over the conductive film 504 a, theinsulating film 502, the insulating film 500, the conductive film 114 d,and the conductive film 114 c, and a resist mask is formed over theinsulating film by a photolithography process. After that, part of theinsulating film is etched using the resist mask to form an insulatingfilm 510 (FIG. 12D). As illustrated in FIG. 12D, an opening 511 a isformed in part of the insulating film 510 that overlaps the opening 507a and an opening 511 b is formed in part of the insulating film 510 thatoverlaps the conductive film 504 a. In addition, an opening 511 c isformed in part of the insulating film 510 that overlaps the opening 507c.

The insulating film 510 can be formed using a material, a structure, anda method similar to those of the insulating film 104 in Embodiment 1.

<Formation of Conductive Film>

Next, a conductive film is formed over the insulating film 510, theconductive film 504 a, the insulating film 502, the insulating film 500,the conductive film 114 d, and the conductive film 114 c, and a resistmask is formed over the conductive film by a photolithography process.After that, part of the conductive film is etched using the resist maskto form a conductive film 512 a and a conductive film 512 b (FIG. 12E).

The conductive film 512 a is electrically connected to the conductivefilm 114 d and the conductive film 504 a. The conductive film 512 b iselectrically connected to the conductive film 114 c. The conductive film512 b can function as a pixel electrode in a liquid crystal displaydevice. In addition, although not illustrated, a portion where theconductive film 504 a and the conductive film 512 b overlap each othercan function as a capacitor. However, the capacitor is not limitedthereto, and a portion other than the portion where the conductive film504 a and the conductive film 512 b overlap each other can function as acapacitor. For example, a portion where the conductive film 504 a and aconductive film formed separately overlap each other may function as acapacitor.

The conductive film 512 a and the conductive film 512 b can be formedusing a material, a structure, and a method similar to those of theconductive film 504 a.

Through the above-described steps, the connection portion 550 and theconnection portion 560 can be formed.

In the manufacturing method 5, as described above, the use of thehalf-tone photomask enables the openings 507 a and 507 c and theconductive film 504 a to be formed using one mask.

The above-described steps can reduce the number of masks and the numberof photolithography processes. Thus, the manufacturing time andmanufacturing cost of the semiconductor device can be reduced.

Embodiment 3

Another embodiment of the method for manufacturing a semiconductordevice that is one embodiment of the present invention, which isdescribed in Embodiment 2, is described with reference to FIGS. 1A to 1Cand FIGS. 15A to 15C. FIGS. 1A to 1C are top views illustrating thesemiconductor device as described above. FIGS. 15A to 15C eachillustrate the cross section X1-X2 taken along the dashed dotted lineX1-X2 in FIG. 1A. The cross section X1-X2 corresponds to across-sectional structure of part of a region where a transistor and aconductive film provided over the transistor are electrically connectedto each other. FIGS. 15A to 15C each also illustrate the cross sectionY1-Y2 taken along the dashed dotted line Y1-Y2 in FIG. 1B. The crosssection Y1-Y2 corresponds to a cross-sectional structure of part of aregion where a conductive film provided under a semiconductor film and aconductive film provided over the semiconductor film are electricallyconnected to each other. FIGS. 15A to 15C each also illustrate the crosssection Z1-Z2 taken along the dashed dotted line Z1-Z2 in FIG. 1C. Thecross section Z1-Z2 corresponds to a cross-sectional structure of partof a region where a conductive film formed at the same time asconductive films that can function as a source electrode and a drainelectrode of the transistor is electrically connected to two conductivefilms formed above the transistor.

The method for manufacturing a semiconductor device that is described inthis embodiment can be applied to a case where a plurality of conductivefilms are formed above a transistor. For example, the method is suitablefor a liquid crystal display device in an in-plane-switching (IPS) modeor a fringe field switching (FFS) mode where a common electrode and apixel electrode are formed on the same substrate.

<<Manufacturing Method 6>>

A method for forming a connection portion 650 and a connection portion660, which are illustrated in FIG. 15C, is described with reference toFIGS. 15A to 15C. The steps up to and including the formation of theconductive film 504 a in the method for manufacturing a semiconductordevice that is one embodiment of the present invention, which isdescribed in this embodiment, are similar to those in the manufacturingmethod 5 in Embodiment 2; therefore, the manufacturing method 5 can bereferred to for the steps up to and including the formation of theconductive film 504 a. FIG. 15A is a cross-sectional view illustrating astate after the steps up to the formation of the conductive film 504 a.The conductive film 504 a, the opening 507 a, and the opening 507 c areformed using one multi-tone mask (a half-tone photomask or a gray-tonephotomask), resulting in the reduced numbers of masks andphotolithography processes. Thus, the manufacturing time andmanufacturing cost of the semiconductor device can be reduced.

<Formation of Insulating Film>

Next, an insulating film is formed over the conductive film 504 a, theinsulating film 502, the insulating film 500, the conductive film 114 c,and the conductive film 114 d, and a resist mask is formed over theinsulating film by a photolithography process. Then, part of theinsulating film is etched using the resist mask to form an insulatingfilm 610 (FIG. 15B). As illustrated in FIG. 15B, an opening 611 a isformed in the insulating film 610 inside the opening 507 a, and theopening 511 b is formed in part of the insulating film 610 that overlapsthe conductive film 504 a. In addition, an opening 611 c is formedinside the opening 507 c.

Unlike the manufacturing method 5 in Embodiment 2, the insulating film610 is also formed inside the opening 507 a and the opening 507 c. Thus,the side surfaces of the insulating film 502 in the opening 507 a andthe opening 507 c are covered with the insulating film 610. Thisstructure can suppress diffusion of impurities inside the insulatingfilm 502 or impurities and the like attached to the side surfaces of theinsulating film 502 in the opening 507 a and the opening 507 c in a stepsuch as ashing of the multi-tone mask to the outside of the insulatingfilm 502. When the impurities are mixed in liquid crystal in, forexample, a liquid crystal display device described later in Embodiment4, the liquid crystal deteriorates. The suppression of diffusion of theimpurities by the insulating film 502 can prevent the deterioration ofthe liquid crystal display device.

<Formation of Conductive Film>

Next, a conductive film is formed over the insulating film 610, theconductive film 504 a, the insulating film 502, the insulating film 500,the conductive film 114 c, and the conductive film 114 d, and a resistmask is formed over the conductive film by a photolithography process.After that, part of the conductive film is etched using the resist maskto form a conductive film 612 a and a conductive film 612 b (FIG. 15C).

The conductive film 612 a is electrically connected to the conductivefilm 114 d and the conductive film 504 a. The conductive film 612 b iselectrically connected to the conductive film 114 c. The conductive film612 b can function as a pixel electrode in a liquid crystal displaydevice. In addition, although not illustrated, a portion where theconductive film 504 a and the conductive film 612 b overlap each othercan function as a capacitor. However, the capacitor is not limitedthereto, and a portion other than the portion where the conductive film504 a and the conductive film 612 b overlap each other can function as acapacitor. For example, a portion where the conductive film 504 a and aconductive film formed separately overlap each other may function as acapacitor.

The conductive film 612 a and the conductive film 612 b can be formedusing a material, a structure, and a method similar to those of theconductive film 504 a.

Through the above-described steps, the connection portion 650 and theconnection portion 660 can be formed.

As is understood from the cross-sectional views of FIGS. 15A to 15C, theinsulating film 500, the insulating film 502, and the insulating film610 may have tapered side surfaces in the connection portion 650 and theconnection portion 660. The insulating film 500, the insulating film502, and the insulating film 610 that have the tapered side surfacesmake the conductive film 612 a and the conductive film 612 b also havetapered shapes in the connection portion 650 and the connection portion660. When the insulating film 500, the insulating film 502, and theinsulating film 610 have upright side surfaces in the connection portion650 and the connection portion 660, the conductive film 612 a and theconductive film 612 b cannot be formed sufficiently, resulting indisconnection of the conductive film 612 a and the conductive film 612b. In contrast, when the insulating film 500, the insulating film 502,and the insulating film 610 have the tapered side surfaces, thedisconnection of the conductive film 612 a and the conductive film 612 bcan be prevented, resulting in the improved reliability of thesemiconductor device.

As described above, in the manufacturing method 6, the use of thehalf-tone mask enables the opening 507 a, the opening 507 c, and theconductive film 504 a to be formed using one mask. In addition,diffusion of impurities from the side surfaces of the insulating film502 can be suppressed.

The above-described steps can reduce the number of masks and the numberof photolithography processes. Thus, the manufacturing time andmanufacturing cost of the semiconductor device can be reduced. Inaddition, the semiconductor device can be used without a decrease in thereliability.

Embodiment 4

In this embodiment, a semiconductor device such as a display device, towhich any of the methods for manufacturing a semiconductor devicedescribed in Embodiments 1 to 3 can be applied, is described.

The manufacturing method described in Embodiment 1 can be suitably usedas a method for manufacturing a switching element of an active matrixliquid crystal display device and can be applied to liquid crystaldisplay devices in a variety of modes such as a twisted nematic (TN)mode, a vertical alignment (VA) mode, and an IPS mode.

The manufacturing method described in Embodiment 1 can also be suitablyused as a method for manufacturing a switching element of alight-emitting device including an active matrix organic EL element.

The manufacturing method described in Embodiment 2 and Embodiment 3 canbe suitably used as a method for manufacturing a switching element of,particularly, a liquid crystal display device in an IPS mode. In thiscase, a pixel electrode having slits is formed. The stacking order ofthe pixel electrode and the common electrode in Embodiment 2 andEmbodiment 3 may be switched to form a common electrode having slits.

The manufacturing methods described in Embodiments 1 to 3 can besuitably used as a method for manufacturing a semiconductor device suchas a memory device, an arithmetic device, a CPU, or a microcomputer,without limitation to a display device and a light-emitting device.

A liquid crystal display device 1000 in an IPS (FFS) mode, in which thesemiconductor device described in Embodiment 1 is used, is describedbelow as an example of a semiconductor device with reference to FIGS.13A to 13C.

FIG. 13A is a top view of the liquid crystal display device 1000. FIG.13B illustrates the cross section M1-M2 taken along the dashed lineM1-M2 in FIG. 13A. The cross section M1-M2 corresponds to across-sectional structure of a region where the transistor 150 describedin Embodiment 1 is formed, and the region can function as a pixel regionof the liquid crystal display device. FIG. 13C illustrates the crosssection N1-N2 taken along the dashed line N1-N2 in FIG. 13A. The regionillustrated in the cross section N1-N2 can function as a peripheralportion where a driver and the like are provided and a connectionportion in the liquid crystal display device.

As illustrated in FIG. 13A, the liquid crystal display device 1000includes a pixel portion 1201, a driver 1200, and a driver 1202 betweena substrate 1111 and a substrate 1121.

As illustrated in FIGS. 13B and 13C, the liquid crystal display device1000 also includes the transistor 150 over the substrate 1111. Theliquid crystal display device 1000 also includes a polarizing plate1161, a liquid crystal layer 1115, a substrate 1121, a touch panelportion 1100, a bonding layer 1163, a polarizing plate 1162, a substrate1131, and the like. The touch panel portion 1100 includes an electrode1122, an electrode 1123, and the like. In addition, an alignment film1155 and an alignment film 1156 are provided over and under the liquidcrystal layer, and a spacer 1165 is provided in order to retain a cellgap of the liquid crystal layer. In addition, a color filter 1114 isprovided in a portion that does not overlap the transistor 150, and aconductive film 1113 and a conductive film 1235 are formed to overlapthe color filter 1114.

In a terminal portion, the substrate 1121 and the substrate 1111 arebonded to each other by a scaling material 1421. An electrode 1122 andan FPC 1433 are electrically connected to each other through aconductive film 1431 and a conductive film 1432. A conductive film 1422and an FPC 1424 are electrically connected to each other through aconductive film 1423.

Although the structure in which the insulating film 502 that canfunction as a planarization film is provided is illustrated in FIGS. 13Ato 13C, one embodiment of the present invention is not limited to thestructure. A display device may have a structure in which the insulatingfilm 502 is not provided.

Embodiment 5

In this embodiment, examples of an electronic appliance including aliquid crystal display device to which one embodiment of the presentinvention is applied are described with reference to FIGS. 14A to 14E.

Electronic appliances described in this embodiment each include theliquid crystal display device of one embodiment of the present inventionin a display portion.

Examples of an electronic appliance including the liquid crystal displaydevice are television devices (also referred to as televisions ortelevision receivers), monitors of computers or the like, digitalcameras, digital video cameras, digital photo frames, mobile phone sets(also referred to as mobile phones or mobile phone devices), portablegame machines, portable information terminals, audio reproducingdevices, and large-sized game machines such as pachinko machines.Specific examples of these electronic appliances are illustrated inFIGS. 14A to 14E.

FIG. 14A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7102 is incorporated in a housing 7101.Images can be displayed on the display portion 7102. The liquid crystaldisplay device to which one embodiment of the present invention isapplied can be used for the display portion 7102. In addition, here, thehousing 7101 is supported by a stand 7103.

The television device 7100 can be operated with an operation switch ofthe housing 7101 or a separate remote controller 7111. With operationkeys of the remote controller 7111, channels and volume can becontrolled and images displayed on the display portion 7102 can becontrolled. The remote controller 7111 may be provided with a displayportion for displaying data output from the remote controller 7111.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 14B illustrates an example of a computer. A computer 7200 includesa main body 7201, a housing 7202, a display portion 7203, a keyboard7204, an external connection port 7205, a pointing device 7206, and thelike. The computer is manufactured using the liquid crystal displaydevice of one embodiment of the present invention for the displayportion 7203.

FIG. 14C illustrates an example of a portable game machine. A portablegame machine 7300 has two housings, a housing 7301 a and a housing 7301b, which are connected with a joint portion 7302 so that the portablegame machine can be opened and closed. The housing 7301 a incorporates adisplay portion 7303 a, and the housing 7301 b incorporates a displayportion 7303 b. In addition, the portable game machine illustrated inFIG. 14C includes a speaker portion 7304, a recording medium insertionportion 7305, an operation key 7306, a connection terminal 7307, asensor 7308 (a sensor having a function of measuring or sensing force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, electriccurrent, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), an LED lamp, amicrophone, and the like. It is needless to say that the structure ofthe portable game machine is not limited to the above structure as longas the liquid crystal display device of one embodiment of the presentinvention is used for at least either the display portion 7303 a or thedisplay portion 7303 b, or both, and may include other accessories asappropriate. The portable game machine illustrated in FIG. 14C has afunction of reading out a program or data stored in a recoding medium todisplay it on the display portion, and a function of sharing informationwith another portable game machine by wireless communication. Note thata function of the portable game machine illustrated in FIG. 14C is notlimited to them, and the portable game machine can have a variety offunctions.

FIG. 14D illustrates an example of a mobile phone. A mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,an operation button 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured using the liquid crystal display device of oneembodiment of the present invention for the display portion 7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 14D is touched with a finger or the like, data can be input to themobile phone 7400. In addition, operations such as making a call andcreating e-mail can be performed by touch on the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as characters. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of making a call or creating e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on the screen can beinput.

When a sensing device including a sensor such as a gyroscope sensor oran acceleration sensor for sensing inclination is provided inside themobile phone 7400, display on the screen of the display portion 7402 canbe automatically changed in direction by determining the orientation ofthe mobile phone 7400 (whether the mobile phone 7400 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are changed by touch on the display portion 7402 oroperation with the operation button 7403 of the housing 7401. The screenmodes can be switched depending on the kind of images displayed on thedisplay portion 7402. For example, when a signal of an image displayedon the display portion is a signal of moving image data, the screen modeis switched to the display mode. When the signal is a signal of textdata, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, when a backlight or asensing light source which emits near-infrared light is provided in thedisplay portion, an image of a finger vein, a palm vein, or the like canbe taken.

FIG. 14E illustrates an example of a foldable tablet terminal (which isunfolded). A tablet terminal 7500 includes a housing 7501 a, a housing7501 b, a display portion 7502 a, and a display portion 7502 b. Thehousing 7501 a and the housing 7501 b are connected by a hinge 7503 andcan be opened and closed using the hinge 7503 as an axis. The housing7501 a includes a power switch 7504, operation keys 7505, a speaker7506, and the like. Note that the tablet terminal 7500 is manufacturedusing the liquid crystal display device of one embodiment of the presentinvention for either the display portion 7502 a or the display portion7502 b, or both.

Part of the display portion 7502 a or the display portion 7502 b can beused as a touch panel region, where data can be input by touchingdisplayed operation keys. For example, a keyboard can be displayed onthe entire region of the display portion 7502 a so that the displayportion 7502 a is used as a touch screen, and the display portion 7502 bcan be used as a display screen.

This embodiment can be combined with any of the other embodiments asappropriate.

This application is based on Japanese Patent Application serial no.2013-078908 filed with the Japan Patent Office on Apr. 4, 2013, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming a first conductive film over asubstrate; forming a first insulating film over the first conductivefilm; forming a semiconductor film over the first insulating film;etching the semiconductor film; forming a second insulating film overthe semiconductor film; forming a mask over the second insulating film;removing first portions of the first insulating film and the secondinsulating film with the use of the mask to form a first opening, andremoving a second portion of the second insulating film with the use ofthe mask to form a second opening; removing the mask; and forming asecond conductive film over the second insulating film, the secondconductive film electrically connected to the semiconductor film throughthe second opening, wherein the semiconductor film and the firstportions do not overlap each other, and the semiconductor film and thesecond portion overlap each other, and wherein the semiconductor filmincludes a channel region.
 2. The method for manufacturing asemiconductor device, according to claim 1, further comprising the stepsof: receding the mask to expose a part of the second insulating filmbetween the removal of the second portion and the first portions and theremoval of the mask; and removing the part of the second insulatingfilm.
 3. The method for manufacturing a semiconductor device, accordingto claim 1, wherein the semiconductor film includes an oxidesemiconductor.
 4. A method for manufacturing a semiconductor device,comprising the steps of: forming a first conductive film over asubstrate; forming a first insulating film over the first conductivefilm; forming a semiconductor film over the first insulating film;etching the semiconductor film; forming a second insulating film overthe semiconductor film; forming a first mask including a first regionand a second region over the second insulating film, wherein a thicknessof the first mask in the second region is smaller than a thickness ofthe first mask in the first region; removing first portions of the firstinsulating film and the second insulating film with the use of the firstmask; removing the first mask in the second region to form a second maskafter removing the first portions; removing a second portion of thesecond insulating film with the use of the second mask; removing thesecond mask after removing the second portion; and forming a secondconductive film over the second insulating film, the second conductivefilm electrically connected to the semiconductor film, wherein thesemiconductor film includes a channel region.
 5. The method formanufacturing a semiconductor device, according to claim 4, wherein thesemiconductor film includes an oxide semiconductor.
 6. A method formanufacturing a semiconductor device, comprising the steps of: forming afirst conductive film over a substrate; forming a first insulating filmover the first conductive film; forming a semiconductor film over thefirst insulating film; forming a first mask including a first region anda second region over the semiconductor film, wherein a thickness of thefirst mask in the second region is smaller than a thickness of the firstmask in the first region; removing first portions of the firstinsulating film and the semiconductor film to form a first opening withthe use of the first mask; removing the first mask in the second regionto form a second mask after removing the first portions; removing asecond portion of the semiconductor film with the use of the secondmask; removing the second mask after removing the second portion;forming a second insulating film over the semiconductor film; removing afirst portion of the second insulating film to form a second opening inthe first insulating film and the second insulating film, and a secondportion of the second insulating film to form a third opening in thesecond insulating film, the first portion of the second insulating filmand the first opening overlap each other; and forming a secondconductive film over the second insulating film, the second conductivefilm electrically connected to the semiconductor film through the thirdopening, wherein the semiconductor film includes a channel region. 7.The method for manufacturing a semiconductor device, according to claim6, wherein the semiconductor film includes an oxide semiconductor.